Network Working Group L. Andersson
Request for Comments: 5462 Acreo AB
Updates: 3032, 3270, 3272, 3443, 3469, R. Asati
3564, 3985, 4182, 4364, 4379, Cisco Systems
4448, 4761, 5129 February 2009
Category: Standards Track
Multiprotocol Label Switching (MPLS) Label Stack Entry:"EXP" Field Renamed to "Traffic Class" Field
Status of This Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
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Abstract
The early Multiprotocol Label Switching (MPLS) documents defined the
form of the MPLS label stack entry. This includes a three-bit field
called the "EXP field". The exact use of this field was not defined
by these documents, except to state that it was to be "reserved for
experimental use".
Although the intended use of the EXP field was as a "Class of
Service" (CoS) field, it was not named a CoS field by these early
documents because the use of such a CoS field was not considered to
be sufficiently defined. Today a number of standards documents
define its usage as a CoS field.
Andersson & Asati Standards Track [Page 1]

RFC 5462 MPLS TC Field Definition February 2009
1. The use of the EXP field was first defined in RFC 3270 [RFC3270],
where a method to define a variant of Diffserv Label Switched
Paths (LSP), called EXP-Inferred-PSC LSP (E-LSPs), was specified.
PSC is a two-stage acronym that is expanded as PHB (Per Hop
Behavior) Scheduling Class (PSC).
2. The use of the EXP field as defined in RFC 3270 has been further
extended in RFC 5129 [RFC5129], where methods for explicit
congestion marking in MPLS are defined.
This document, hence, uses the name "Traffic Class field (TC field)",
which better covers the potential use. The MPLS TC field relates to
an MPLS encapsulated packet the same way as the IPv6 TC field relates
to an IPv6 encapsulated packet or the IPv4 Precedence field relates
to an IPv4 encapsulated packet.
The definitions of how the EXP field is used are perfectly clear in
RFC 3270 and RFC 5129. However, these RFCs do not explicitly state
they update RFC 3032, and this fact was not captured in the RFC
repository until after work on this document was started.
This document updates RFC 3032, RFC 3270, and RFC 5129 to clarify the
intended usage of the TC field. The changes to these RFCs requires
some changes to the actual text in those documents; Section 2
explains the changes.
This document also updates several other RFCs; see Section 2.4. For
these documents, the change is limited to changing the name of the
Label Stack entry field.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Details of Change
The three RFCs 3032, 3270, and 5129 are now updated according to the
following.
2.1. RFC 3032RFC 3032 states on page 4:
3. Experimental Use
This three-bit field is reserved for experimental use.
Andersson & Asati Standards Track [Page 3]

RFC 5462 MPLS TC Field Definition February 20092.2. RFC 3270RFC 3270 says on page 6:
1.2 EXP-Inferred-PSC LSPs (E-LSP)
A single LSP can be used to support one or more OAs. Such LSPs
can support up to eight BAs of a given FEC, regardless of how many
OAs these BAs span. With such LSPs, the EXP field of the MPLS
Shim Header is used by the LSR to determine the PHB to be applied
to the packet. This includes both the PSC and the drop
preference.
We refer to such LSPs as "EXP-inferred-PSC LSPs" (E-LSP), since
the PSC of a packet transported on this LSP depends on the EXP
field value for that packet.
The mapping from the EXP field to the PHB (i.e., to PSC and drop
precedence) for a given such LSP, is either explicitly signaled at
label set-up or relies on a pre-configured mapping.
Detailed operations of E-LSPs are specified in section 3 below.
RFC 3270 is now updated like this:
a. A new paragraph is added at the end of Section 1 "Introduction":
The EXP field has been renamed the TC field, and thus all
references in RFC 3270 to the EXP field now refer to the TC
field.
b. A new term is added to Section 1.1 "Terminology":
TC Traffic Class (replaces the term EXP)
c. In Section 1.1 "Terminology", the acronym E-LSP is now understood
to mean:
E-LSP Explicitly TC-encoded-PSC LSP
Section 1.2 on page 6 in RFC 3270 is now changed to:
1.2 Explicitly TC-encoded-PSC LSPs (E-LSP)
The EXP field has been renamed to the TC field, and thus all
references in RFC 3270 to EXP field now refer to the TC field.
However, we retain the acronym E-LSP (Explicitly TC-encoded-PSC
LSP) as the acronym is in widespread use.
Andersson & Asati Standards Track [Page 5]

RFC 5462 MPLS TC Field Definition February 2009
A single LSP can be used to support one or more OAs. Such LSPs
can support up to eight BAs of a given FEC, regardless of how many
OAs these BAs span. With such LSPs, the TC field of the MPLS Shim
Header is used by the LSR to determine the PHB to be applied to
the packet. This includes both the PSC and the drop preference.
We refer to such LSPs as "Explicitly TC-encoded-PSC LSPs"
(E-LSPs), since the PSC of a packet transported on this LSP
depends on the TC field (previously called the EXP field) value
for that packet.
The mapping from the TC field to the PHB (i.e., to PSC and drop
precedence) for a given such LSP is either explicitly signaled at
label set-up or relies on a pre-configured mapping.
This is an update to RFC 3032 [RFC3032], in line with the original
intent of how this field in the MPLS Shim Header should be used
(as a TC field). RFC 3270 has itself been updated by RFC 5129
[RFC5129].
Detailed operations of E-LSPs are specified in Section 3 of RFC3270.
2.3. RFC 5129RFC 5129 is now updated like this:
A new paragraph is added at the end of Section 1.1 "Background":
The EXP field has been renamed to the TC field, and thus all
references in RFC 5129 to the EXP field now refer to the TC field.
Section 2 (bullet 5) on page 7 of RFC 5129 says:
o A third possible approach was suggested by [Shayman]. In this
scheme, interior LSRs assume that the endpoints are ECN-capable,
but this assumption is checked when the final label is popped. If
an interior LSR has marked ECN in the EXP field of the shim
header, but the IP header says the endpoints are not ECN-capable,
the edge router (or penultimate router, if using penultimate hop
popping) drops the packet. We recommend this scheme, which we
call `per-domain ECT checking', and define it more precisely in
the following section. Its chief drawback is that it can cause
packets to be forwarded after encountering congestion only to be
dropped at the egress of the MPLS domain. The rationale for this
decision is given in Section 8.1.
Andersson & Asati Standards Track [Page 6]

RFC 5462 MPLS TC Field Definition February 2009Section 2 (bullet 5) of RFC 5129 is now updated to:
o A third possible approach was suggested by [Shayman]. In this
scheme, interior LSRs assume that the endpoints are ECN-capable,
but this assumption is checked when the final label is popped. If
an interior LSR has marked ECN in the TC field of the shim header,
but the IP header says the endpoints are not TC-capable, the edge
router (or penultimate router, if using penultimate hop popping)
drops the packet. We recommend this scheme, which we call `per-
domain ECT checking', and define it more precisely in the
following section. Its chief drawback is that it can cause
packets to be forwarded after encountering congestion only to be
dropped at the egress of the MPLS domain. The rationale for this
decision is given in Section 8.1. This scheme is an update to RFC3032 [RFC3032] and RFC 3270 [RFC3270].
2.4. The Scope of This Change
There are several places in the RFCs that are explicitly updated by
this document that reference the "Exp field", sometimes they refer to
the field as "Exp bits", "EXP bits", or "EXP". In all those
instances, the references now reference the TC field.
There are also other RFCs (e.g., RFC 3272 [RFC3272], RFC 3443
[RFC3443], RFC 3469 [RFC3469], RFC 3564 [RFC3564], RFC 3985
[RFC3985], RFC 4182 [RFC4182], RFC 4364 [RFC4364], RFC 4379
[RFC4379], RFC 4448 [RFC4448], and RFC 4761 [RFC4761]) that reference
the "Exp field"; sometimes they refer to the field as "Exp bits",
"EXP bits", and "EXP". For all RFCs, including but not limited to
those mentioned in this paragraph, such references now reference the
TC field.
3. Use of the TC field
Due to the limited number of bits in the TC field, their use for QoS
and ECN (Explicit Congestion Notification) functions is intended to
be flexible. These functions may rewrite all or some of the bits in
the TC field.
Current implementations look at the TC field with and without label
context, and the TC field may be copied to the label stack entries
that are pushed onto the label stack. This is done to avoid label
stack entries that are pushed onto an existing label stack having
different TC fields from the rest of the label stack entries.
Andersson & Asati Standards Track [Page 7]